1. Field of the Invention
This invention relates to rotary helical screw compressors incorporating a slide valve for controlling compressor capacity, and more particularly to a slide valve feedback control system which is responsive to compressor discharge or line pressure.
2. Description of the Prior Art
In a helical, rotary screw compressor, the intermeshed helical screws acting in conjunction with the fixed compressor housing define the compressor working chamber in terms of closed threads, and the compressor operates as a positive displacement machine for compressing air, or a gas such as a refrigerant between the suction and discharge sides of the screw compressor. The match compressor load during compressor operation, the capacity of the helical, rotary screw compressor has been varied by incorporating a capacity control slide valve within the housing and slidable parallel to the axis of the screws. The slide valve shifts longitudinally between limits to control the percentage of the working fluid which is passed from the inlet or suction side to the discharge side of the machine. Conventionally, with the slide valve in closed position and against a valve stop, the compressor is fully loaded, in which case all of the working fluid flows from the intake to the discharge side. Unloading is achieved by moving the slide valve away from the valve stop to create a bypass which returns a portion of the suction gas to the inlet port area prior to compression of the same. Enlarging of the opening in the rotor housing by shifting of the valve longitudinally reduces the compressor displacement.
Screw compressors with slide valve unloading encounter load response problems based on system demand, because of:
(1) THE INHERENTLY HIGH AND SOMEWHAT VARIABLE FRICTION FORCES OPPOSING SLIDE VALVE MOVEMENT;
(2) OVERSHOOTING OF THE SLIDE VALVE IF SLIDE VALVE ACTUATION IS FAST BECAUSE OF THE TIME LAG IN SYSTEM PRESSURE CHANGES WHEN DEMAND AND CAPACITY ARE VARIED, RESULTING IN THE COMPRESSOR CONTINUOUSLY EXCESSIVELY HUNTING; AND
(3) NONRESPONSIVENESS OF THE SLIDE VALVE TO QUICK CHANGES IN DEMAND, IF SLIDE VALVE ACTUATION IS SLOWED DOWN IN AN ATTEMPT TO REDUCE HUNTING.
Screw compressors with slide valve unloading have a propoportional power reduction with reduction in load but have a power input of about forty-five to fifty-five percent of full load power when operated at minimum load conditions for extended periods of time, in such systems where the working fluid which may be air or other gas, is discharging into a gas storage tank which is maintained at a given pressure. This invention will be described in conjunction with an air compressor system, wherein the compressor functions to maintain a given pressure to air stored within a tank for delivery to a load dependent upon load demand.
FIG. 1 constitutes a graphical illustration or plot of power against load for an air compressor operating to maintain 100 to 110 psi tank pressure in a pressurized air system with the power requirement for a conventional screw compressor system given by the solid line A for varying compressor load. With the compressor acting against a tank pressure of 100 psi, at zero load, the power requirements of the compressor are approximately fifty percent of full load. The present invention aims at reducing power requirements at minimum load as illustrated by the dotted line B of the plot which intersects the solid line A at a point approximating ten percent of compressor load and with the system operating otherwise identical between load conditions of ten percent to one hundred percent.
Reference to FIG. 2 shows the typical prior art compressor unloading arrangement for a helical screw rotary compressor operating within a typical refrigeration system. The compressor illustrated in FIG. 2 comprises schematically, a compressor casing 1 supporting intermeshed screws, one of which is shown at 2, and having a slide valve 3 movable longitudinally relative to the screws for controlling the return of a portion of the working fluid back to the suction side 4 of the machine. The position of the slide valve 3 is controlled by a hydraulic motor 5 incorporating a piston 6 which is directly connected to the slide valve 3 via rod 7. Oil under pressure, as indicated by arrow 8 acting through line 11, is applied to the outboard side or face of piston 6 to unload the compressor which overcomes the discharge gas pressure as indicated by arrows PD, acting on the discharge end of the side valve 3. When oil is bled from the outboard side of the piston 6, the pressure is reduced and the compressor slide valve 3 begins to load due to the discharge gas pressure force acting on the slide valve 3. Control of the slide valve is effected by means of solenoid valves SOL.sub.1 and SOL.sub.2 within line 11 open to the outboard side of piston 6 and line 9 leading to suction, respectively.
In a typical control system for such a helical screw compressor operating within a refrigeration or air conditioning system, a signal indicative of rise in suction pressure acts to open, normally closed solenoid valve SOL.sub.1 in which case, oil drains from the outboard side of the piston in the direction of arrow 12 to conmpressor suction 4 and the compressor slide valve 3 shifts under the applied discharge gas pressure by way of arrows 10, the valve moving to the left towards load position. To the contrary, in response to compressor suction pressure drop, an appropriate circuit is completed to solenoid valve SOL.sub.2, this valve delivering oil under pressure from a source, as per arrow 8, to the outboard side of the piston 6, forcing the slide valve 3 to move to the right against the discharge gas pressure PD and unloading the compressor.
In attempts to employ the unloading arrangement of FIG. 2 to an air compressor or similar application where discharge or system pressure is employed to activate solenoid valves SOL.sub.1, and SOL.sub.2, certain problems arise. The time lag in system pressure in reflecting the change in capacity will cause the slide valve to overshoot its desired position unless its actuation time is slowed down, which then results in the compressor capacity not responding fast enough to meet large and quick changes in system demand. Further, since the discharge gas pressure furnishes the loading force, start up with the slide valve in the zero load position is near impossible since the compressor never loads and there never is a force (indicated by arrows PD) acting on the discharge side of the slide valve in opposition to the applied oil pressure. Further, if not impossible, the system would take a long time to load, the system pressure being built up slowly with an unloaded compressor. Once built up, the system pressure could position the slide valve.
It is, therefore, an object of this invention to provide a system in which the unloading control is insensitive to high and variable slide valve friction. It is a further object of this invention to provide an improved slide valve unloading control system for a helical screw compressor which employs a mechanical feedback of the slide valve position compared with system demand requirements.
Where the rotary helical screw compressor to which the unload control system of the present invention is employed constitutes an air compressor and wherein compressed air is stored at a given pressure for load application, it is a further object of this invention to provide the system with means for dumping the tank when the compressor is operating at minimum load such that the compressor continues on the line, but operates close to zero psi discharge pressure to reduce the minimum load horse power requirements to less than ten percent of that of full load.